The Distinct and Surprisingly Diverse Populations and Properties of Mars Mesospheric Aerosols
Résumé
Analysis of a 2009-2016 set of MRO CRISM limb observations and contemporaneous MCS and MARCI observations yields a new and uniquely comprehensive characterization of dust and ice aerosol distributions and physical characteristics in the Mars mesosphere (50-100 km). Key conclusions are: [1] very distinct aphelion (Ls=0-160º) and perihelion (Ls=160-360º) aerosol populations, in which low latitude CO2 clouds at 60-70 km altitudes dominate the aphelion mesosphere and H2O ice clouds at 50-75 km altitudes dominate the perihelion mesosphere; [2] mesospheric H2O clouds composed of small particle sizes (Reff=0.2 μm) and CO2 cloud particles exhibiting a very broad range of particle sizes (Reff=0.3-2.5 μm) with small particle sizes prominent at the latitude/altitude boundaries of mesospheric CO2 cloud formation; [3] infrequent mesospheric dust aerosols in these non-planet encircling dust storm years with small particle sizes (Reff=0.3-0.6 μm); [4] MCS temperature measurements indicating saturation conditions in the vicinity of mesospheric ice aerosols and enhanced solar heating in the presence of dust aerosols; [5] very narrow CO2 particle size distributions (Veff=0.01-0.03) indicative of iridescence and rapid, uniform cloud nucleation; [6] MCS-CRISM comparisons indicating comparable daytime (3pm) CO2 cloud particle sizes but also MCS nighttime (3am) measurements indicating pervasive low latitude mesospheric CO2 clouds at lower altitudes (55-65 km) and with larger particle sizes (Reff=4-6 μm); [7] MARCI imaging of mesospheric ice clouds displaying wave forms indicative of gravity-wave forcing; and [8] strong indications that cloud nucleation centers, whatever they may be, are not lacking in Mars' mesosphere.